1. Field of the Invention
The present invention relates to a trench MOSFET structure with a guard ring and the method for manufacturing the same, and more particularly to a structure of a trench MOSFET which solves a problem of low breakdown voltage in trench gate area and the method for manufacturing the same.
2. Description of the Prior Art
In the structure of a trench Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) or vertical transistor, the gate of the transistor is formed in a trench on top of a substrate and the source/drain regions are formed on both sides of the gate. This type of vertical transistor allows high current to pass through and channel to be turned on/off at a low voltage.
However, such structure is known to have low breakdown voltage occurring on the trench bottom. When reverse bias between the drain and gate/source increases, avalanche will first occur on the trench bottom of the trenched contact gate.
One possible solution to the above problem is to construct a doped region, called a guard ring. The trench MOS device is as a result protected by the native bias of different doping regions. Another approach is to construct a thick field oxide and N+ epitaxial region for the formation of the guard ring. In one aspect, the protection of the guard ring is not adequate. In another aspect, the formation of the thick field oxide requires additional steps.